Son: "Daddy, I fell in love and want to date this awesome girl."
Father: "That's great, son! Who is she?"

Son: "It's Sandra, the neighbor's daughter."
Father: "Ohhh, I wish you hadn't said that. I have to tell you something, son, but you must promise not to tell your mother. Sandra is actually your sister."

The boy is naturally bummed out, but a couple of months later:

Son: "Daddy, I fell in love again and she is even hotter!"
Father: "That's great, son! Who is she?"

Son: "It's Angela, the other neighbor's daughter."
Father: "Ohhh, I wish you hadn't said that. Angela is also your sister."

This went on a few more times, and finally the son was so mad, he went straight to his mother crying.

Son: "Mom, I am so mad at dad! I fell in love with six girls and I can't date any of them because dad is their father!"
The mother hugs him affectionately and says, "You can date whoever you want. He isn't your father!"